1. Field of the Invention
The present invention relates to a thin film magnetic head comprising a magneto-resistive effect element for reading as a signal a magnetic field strength of a magnetic recording medium or the like, and further relates to a head gimbal assembly and a hard disk drive each including such a thin film magnetic head.
2. Description of the Related Art
In recent years, following the improvement in areal recording density of a hard disk drive, improvement in performance of a thin film magnetic head has been required. As the thin film magnetic head, use has been widely made of a composite thin film magnetic head having a structure wherein a reproducing head comprising a read-only magneto-resistive effect element (hereinafter may also be referred to simply as “MR element”), and a recording head comprising a write-only induction-type electromagnetic transducer element are stacked relative to a substrate.
As the MR element, there can be cited an AMR element using an anisotropic magneto-resistive effect, a GMR element using a giant magneto-resistive effect, a TMR element using a tunnel-type magneto-resistive effect, or the like.
As the GMR element, a spin-valve GMR element has been often used. The spin-valve GMR element comprises a nonmagnetic layer, a soft magnetic layer formed on one surface of the nonmagnetic layer, a ferromagnetic layer formed on the other surface of the nonmagnetic layer, and a pinning layer (generally an antiferromagnetic layer) formed on the ferromagnetic layer on its side apart from the nonmagnetic layer. The soft magnetic layer is a layer that acts to change its magnetization direction depending on a signal magnetic field from the exterior. The ferromagnetic layer is a layer of which a magnetization direction is fixed by a magnetic field from the pinning layer (antiferromagnetic layer).
Large output and small Barkhausen noise are required as characteristics of the reproducing head. Generally, in order to reduce the Barkhausen noise, a bias magnetic field is applied to the MR element in a longitudinal direction (hereinafter, this bias magnetic field will be referred to as “longitudinal bias magnetic field”). The application of the longitudinal bias magnetic field to the MR element is carried out by, for example, disposing bias magnetic field applying layers each in the form of a permanent magnet, a stacked body of a ferromagnetic layer and an antiferromagnetic layer, or the like on both sides of the MR element.
In the case of applying the longitudinal bias magnetic field using the permanent magnets, use has generally been made of CoCrPt obtained by adding Cr to CoPt, in view of its large coercive force and high corrosion resistance. Further, as a material of an underlayer employed when forming a bias magnetic field layer of the permanent magnet, use is made of Cr or CrTi that can rest c-axes of CoCrPt of a hexagonal system in the film surface.
WO97/11458 (Patent Document 1) proposes that, in order to change orientation of crystal axes of a permanent magnet film to increase horizontal magnetization components in the film surface and in order to reduce the influence of a magneto-resistive effect film over the property of the permanent magnet film to uniform the property of the permanent magnet film, a ferromagnetic underfilm containing Fe and Cr as main components is placed at a tapered portion of an element and the permanent magnet film is formed thereon.
On the other hand, JP-A-H11-250419 (Patent Document 2) proposes that an underlayer of an element is made of at least one metal selected from a group of Cr, Ta, and Ti. It is described that the selection of such an underfilm is based on the fact that those metals each have a high melting point and high adhesion with a permanent magnet film or the like in manufacturing. The element disclosed in this publication is not of a spin-valve element structure, and there is no specific description about a stacked film structure and moreover there is no specific study for optimization of the thickness of the underlayer or the permanent magnet film.
Generally, when a magnetic field produced from bias magnetic field applying layers is large, the Barkhausen noise is reduced, which is thus preferable. Simultaneously, variation in reproduction waveform symmetry is suppressed. However, there is a problem that if an unnecessarily strong bias magnetic field is applied, the reproduction output is degraded.
In the current development trend, following track narrowing of reproducing heads and reduction in thickness of free layers in response to increase in density of hard disks, reduction in thickness of bias magnetic field applying layers has also been developed. Therefore, there has been required a film design around the bias magnetic field applying layers such that the optimum bias magnetic field can be achieved in response to the reduction in thickness of the whole element.
The present invention has been conceived under these circumstances and has an object to, by performing a film design around bias magnetic field applying layers so that an optimum bias magnetic field can be achieved in response to reduction in thickness of a whole element, provide a thin film magnetic head that produces a large reproduction output even with thin bias magnetic field applying layers, that is excellent in reproduction waveform symmetry with a little variation in reproduction waveform symmetry, and that achieves a small occurrence rate of the Barkhausen noise, and to further provide a head gimbal assembly and a hard disk drive each comprising such an improved thin film magnetic head.